Current hearing aids are capable of both omnidirectional (OMNI) and directional (DIR) processing and newer implementations of OMNI/DIR hearing aids automatically switch between the two microphone processing modes. Both OMNI and DIR processing offer benefits relative the other mode, depending upon the specific listening situation.
For relatively quiet listening situations, OMNI processing is typically preferred over the DIR mode. This is due to the fact that in situations, where any background noise present is fairly low in amplitude, the OMNI mode should provide a greater access to the full range of sounds in the surrounding environment, which may provide a greater feeling of “connectedness” to the environment. The general preference for OMNI processing when the signal source is to the side or behind the listener is predictable. By providing greater access to sound sources that the listener is not currently facing, OMNI processing will improve recognition for speech signals arriving from these locations (e.g., in a restaurant where the server speaks from behind or from the side of listener). This benefit of OMNI processing for target signals arriving from locations other than in front of the listener will be present in both quiet and noisy listening situations. For noisy listening conditions where the listener is facing the signal source (e.g., the talker of interest), the increased SNR provided by DIR processing for signals coming from the front is likely to make DIR processing preferred.
Each of the listening conditions just mentioned (in quiet, in noise with the patient facing or not facing the talker) occur frequently in the everyday experience of hearing-impaired listeners (see for example a study reported in Walden, B. E., Surr, R. K., Cord, M. T., and Dyrlund, O. (2004), Predicting hearing aid microphone preference in everyday listening. Journal of the American Academy of Audiology, 15, 365-396). Thus, hearing aid users regularly encounter listening situations where DIR processing will be preferable to the OMNI mode, and vice versa.
Traditionally, commercial implementations of directional processing require manual switching between the OMNI and DIR microphone modes. The user changes processing modes by flipping a toggle switch or pushing a button on the hearing aid to put the device in the preferred mode according to the listening conditions encountered in a specific environment.
A problem with this approach is that listeners may not be aware that a change in mode could be beneficial in a given listening situation if they do not actively switch modes. In addition, the most appropriate processing mode can change fairly frequently in some listening environments and the listener may be unable to conveniently switch modes manually to handle such dynamic listening conditions. Finally, many listeners may find manual switching and active comparison of the two modes burdensome and inconvenient. As a result, they may leave their devices in the default OMNI mode permanently. In a study reported in Cord, M. T., Surr, R. K., Walden, B. E., Olson, L. (2002), Performance of directional microphones in everyday life, Journal American Academy Audiology, 13, 295-307, it is estimated that about one-third of listeners fitted with manually switchable OMNI/DIR hearing aids may leave their instruments in the default mode regardless of the listening situation. Obviously these patients cannot benefit from the (unused) DIR processing mode.
Recently, several hearing aid manufacturers have introduced hearing aids that automatically switch between OMNI and DIR microphone modes based on some analysis of the acoustic environment. Automatic switching avoids many of the problems associated with manual switching mentioned above. Here, acoustic analysis of the input signal is carried out to determine whether OMNI or DIR processing is likely to be preferred, and the device automatically selects the appropriate mode based on the analysis. Examples of hearing aids that are capable of automatically switching between OMNI and DIR microphone modes are described in the below mentioned patent documents.
In WO 2004114722 a binaural hearing aid system with coordinated sound processing is disclosed, where switching between OMNI and DIR microphones is based on environment classification.
EP 0664071 relates to a hearing aid having a microphone switching system that uses directional microphones for a hearing aid apparatus that is used in circumstances where the background noise renders verbal communication difficult. The invention relates also to switching between an omni-directional microphone and a directional microphone system, based on the measured ambient-noise-level.
U.S. Pat. No. 6,327,370 relates to various techniques of automatic switching between OMNI and DIR microphones according to different noise conditions.
These automatic decisions of switching the microphone modes are all more or less based on rules associated with the level of ambient noise and/or whether a modulated signal, such as speech, is present. However, whether directional microphones are chosen manually by the listener or automatically by the hearing instrument, directional microphones perform a lossy coding of the sound (basically a spectral subtraction occurs by phase shifting one of two signals before addition), eliminating spectral information based on the direction of arrival of the sound. Once this information is removed, it is no longer available or retrievable by the hearing instrument or listener.
Thus, one of the major problems with such methods of manual or automatic switching of microphone modes is the elimination of information, which occurs when the hearing instrument is set to a bilateral directional microphone mode, which may be important to the listener. Though the purpose of a directional microphone is to provide a better signal-to-noise ratio for the signal of interest, the decision of what is the signal of interest is ultimately the listener's choice and cannot be decided upon by the hearing instrument. As the signal of interest is assumed to occur in the look direction of the listener (and on-axis to the directional microphone) any signal that occurs outside the look direction of the listener can and will be eliminated by the directional microphone.
This is in compliance with clinical experience, which suggests that automatic switching algorithms like those discussed above and those currently being marketed are not achieving wide acceptance (see for example: Cord, M. T., Surr, R. K., Walden, B. E., Olson, L. (2002). Performance of directional microphones in everyday life. Journal American Academy Audiology, 13, 295-307). Patients generally prefer to switch modes manually rather than rely of the decisions of these algorithms.